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Zebra mussel population decline in St. Croix puzzles scientists

Scientists have noted a surprising drop in the invasive zebra mussel population in the St. Croix River.

Zebra mussels tend to explode in population and threaten native species. The St. Croix River has a rich community of native mussels, and resource managers are worried about the effects a zebra mussel infestation could have. Department of Natural Resources scientists and others are trying to figure out what's behind the sudden decline.

Just like any native species, zebra mussels can experience population swings. DNR researcher Gary Montz has been watching the mussels in the St. Croix and in other lakes and rivers in Minnesota. He said they are very susceptible to weather and water conditions.

A few years ago, a sudden die-off in Lake Zumbro surprised Montz and other researchers.

"Our theory is that the zebra mussels were stressed by hotter water temps, lower water, perhaps dissolved oxygen getting lower in some of the areas they were surviving in," Montz said. "And it just pushed the population over the edge and a lot of them died off."

On the other hand, the St. Croix River experienced high water last summer. Montz said the fast-flowing river may have caught up a lot of the mussel larvae, which float freely in the water for several weeks.

"They were just swept downstream, perhaps down into the Mississippi River, and by the time ready to settle, far, far away from Lake St. Croix area and the parents," Montz said. "You didn't have a lot of adolescent zebra mussels settling down in the Lake St. Croix area to grow up and start reproducing."

Other research shows that surprising numbers of fish are eating zebra mussels, said Michelle Bartz, who studies this for the U.S. Geological Survey.

"They can definitely reduce numbers, we've seen that in the Mississippi River as well, in some areas by 86 percent," Bartz said. "The problem is there aren't going to be enough fish preying on them continuously, they certainly will not wipe them out."

Researchers are interested to see what happens in the St. Croix this year, when it starts out with relatively low water levels. Despite the population fluctuation, experts say we'll never be rid of zebra mussels completely.